Wouldn't that break a lot of existing markup? For instance
having title = "Look "A Quoted String"" in the header.
And how about when we actually want the en- or em-dash?
Now that Lilypond correctly uses the full Unicode character
set I certainly see no reason to arbitrary limit it's use.

indeed.

I guess the real argument is, clean up the documentation so
that all examples actually work when cut-and-pasted.

I think the proper thing is to add a note to the PDF docs that people
should not expect cutting & pasting from the PDF to work. PDF is made
for getting accurate printout, not for transporting ascii text.